Conventionally, an epoxy resin composition has been used from the standpoint that the encapsulating material for an optical semiconductor element such as light-receiving element and light-emitting element must be excellent in transparency, moisture resistance and heat resistance. The epoxy resin is, for example, transfer-molded in a forming mold having placed therein an optical semiconductor element, whereby the optical semiconductor element is formed as a package to obtain an optical semiconductor device (see, for example, JP-A-2006-93277).
Similarly to a semiconductor package other than an optical semiconductor, also in the optical semiconductor package, for the purpose of reducing the size or weight or enhancing the mounting productivity, surface mount technology is rapidly spreading instead of the conventional through hole mount technology. Examples of such a surface mount-type package include a two-way flat package (small outline package (SOP)), a four-way flat package (quad flat package (QFP)) and SON (small outline non-lead).
In the surface mount technology, unlike the through hole mount technology, the package as a whole is exposed to a high-temperature environment up to 260° C. in mounting practice. At this time, moisture absorbed during storage after the production of an optical semiconductor device abruptly vaporizes and expands to generate a large stress. In the case where this stress exceeds the package strength, cracking occurs in the package. In order to prevent such a problem, the optical semiconductor manufacturers take a countermeasure of shipping the optical semiconductor device in a moisture-proof pack or in the field of mounting, adding a step of, for example, heating and drying the optical semiconductor device in an oven before the mounting step. However, there is involved a cost rise due to moisture-proof packing, bad workability due to packing and unpacking, or a large burden due to the cost of heating and drying. The approach in general for solving the problem of cracking of the encapsulating resin due to water vapor includes a method of incorporating a large amount of a high-strength structural material such as filler into the encapsulating resin, but in the usage for an optical semiconductor, from the standpoint of transparency, it is difficult to use the technique of incorporating a large amount of a high-strength structural material such as filler. Also, an approach of increasing the content of an aliphatic group or a phenyl group to reduce the water absorption of the resin per se and thereby raising the soldering resistance effect may be considered. However, since the glass transition temperature (Tg) of the epoxy resin composition is high and the elastic modulus during solder reflow rises, the stress due to vaporization and expansion when performing reflow cannot be relieved and generation of cracking occurs. As for the technique of reducing the elastic modulus during solder reflow, a method of decreasing the glass transition temperature (Tg) of the epoxy resin composition may be considered. However, this method has a problem in that the temperature cycle reliability is significantly low and the product reliability is not satisfied.